1. Field of the Invention
This invention relates generally to constellation shaping, precoding and coding, and more particularly to a method and system of combining turbo trellis coded modulation (Turbo-TCM/TTCM) with constellation shaping with/without preceding.
2. Description of the Prior Art
Constellation shaping is a means for selecting the stream of transmitted symbols in a communication system, according to a criterion of minimum average power under the constraint of a given minimum distance between neighboring points. By doing that, the constituent two-dimensional constellation is “shaped” into a certain form (with a certain probability distribution between the constellation points). Thus, a symbol sequence with a lower average power can be used (or alternatively a sequence with a high inter-symbol distance between neighboring points for a given power constraint). This results in “shaping gain” which may cause the communication system to become more robust to noise and channel impairments, or result in higher achievable data rates.
Two known methods of constellation shaping are trellis shaping and shell mapping. On Gaussian channels with intersymbol interference (ISI), it is often desired to use preceding methods to mitigate channel distortion. Tomlinson-Harashima (TH) precoding is a well known precoding scheme for ISI-channels.
When constellation shaping is used in ISI channels, it is desired to use a combination of shaping and precoding. A method of combining trellis shaping with TH-precoding is called trellis precoding. Combining shell mapping with preceding can be done by incorporating Laroia precoding. It is known in the art that lattice codes can also be combined with trellis shaping and trellis precoding. Thus, trellis coded modulation (TTCM) schemes can be combined with shaping and preceding to achieve coding gain together with shaping gain, and to have high performance, even in ISI-channels.
Lattice codes (e.g. TTCM) can also be combined with shell mapping and Laroia preceding to achieve coding gain together with shaping gain, and to have high performance, even in ISI-channels. This is done for example, in the ITU V.34 standard for voice grade modems.
Recently, a new coding technique, “turbo coding” was introduced, which can be used to achieve higher coding gains than other coding schemes; and when used in communication systems, may result in better robustness to noise and other impairments, or in higher (closer to capacity) data rates. Whereas the original work in the field of turbo coding addresses the case of binary transmission, new schemes have recently been developed for incorporating turbo coding when using larger constellations. These schemes are referred to as Turbo trellis coded modulation (TTCM).
In view of the foregoing, a need exists for a technique that combines a TTCM coding scheme with shaping and preceding schemes to implement a system and method that can achieve better performance (higher coding gains achieved in combination with shaping gain, and when necessary, also with high performance in ISI-channels via precoding) than existing methods and systems.